Laser apparatus requires a source of pumping power to produce excitation of a laser amplifying medium. In one class of lasers, pumping power is provided by a discharge produced between two electrodes in a gas-filled tube. The gas may itself act as the laser amplifying medium, or may comprise a buffer gas in which the discharge occurs and which heats other material within the tube to provide vapour which acts as the amplifying medium. For example, in a metal vapour laser, the discharge is produced in a buffer gas and the heat of the discharge causes vapour to be produced from a solid metal charge within the tube.
To produce a discharge within the discharge tube a suitably large potential difference must be applied between its two electrodes. The breakdown voltage at which a discharge occurs is dependent on the gas pressure within the tube and the distance between the electrodes, in accordance with Paschen's law. The potential difference is applied between the electrodes in the discharge tube by using a circuit arrangement which includes a capacitor and a thyratron. Initially, the thyratron is not conducting and the capacitor is charged. The circuit is designed so that, when it is desired to produce a discharge within the discharge tube, the thyratron is triggered into conduction and the energy stored by the capacitor discharges through the gas in the tube.